Cough reflex is a common important defense mechanism, and effectors thereof are located at larynx, trachea and bronchial mucosa. The effector located above primary bronchus is sensitive to mechanical stimulation, and the effector located below secondary bronchus is sensitive to chemical stimulation. The cough reflex is triggered when afferent impulse is introduced into medulla oblongata.
To cough, a short or deep inhaling (normally 2.5 L) happens firstly; then glottis is closed; expiratory muscle (abdominal muscle and internal intercostal muscle) are contracted intensively; intrapulmonary pressure and intrapleural pressure are increased sharply (may be up to 100 mmHg or even more); then the glottis is opened suddenly; gas inside lung is discharged at high speed as the intrapulmonary pressure is very high, such that foreign matters or secretions inside respiratory tract are discharged.
A patient suffering from myasthenia of abdominal muscle and respiratory muscle has a weakened cough reflex. Due to limited thoracic expansion in a patient lying in bed, gas getting into lung before cough is insufficient, thereby resulting in weakened or even invalid cough. Besides, the cough reflex is also limited in a patient with suppressed medullary center.
Patients with weakened respiratory muscle caused by various clinical reasons often need a ventilator for assisting to breathe. Cough in this kind of patients is weak such that it is impossible to expectorate secretions deep down inside lung effectively, resulting in very high morbidity of ventilator-associated pneumonia. It is very important to assist a patient to expectorate secretions deep down inside lung, so as to prevent the ventilator-associated pneumonia. Currently, a commonly-used method for sputum suction is carried out with a closed sputum suction tube.
The term “closed phlegm suction” refers to a sputum suction operation without disconnecting from the ventilator or stopping mechanical ventilation. Moreover, the sputum suction tube is covered with a transparent membrane, and entire sputum suction operation is completed under closed environment. As there are up to 20 or more levels of bronchus inside lung, the sputum suction tube is merely capable of sucking sputum in the primary bronchial. In addition, as the lung is of an asymmetrical structure, the sputum suction tube is merely capable of sucking sputum in right lung, other than sputum in left lung. For this reason, the ventilator-associated pneumonia usually occurs in the left lung. Furthermore, the sputum suction tube increases risks in causing airway scratch, introducing bacteria and influencing hemodynamics.
Except for such a device for sputum suction (eg. the closed sputum suction tube) manually operated, an automatic device for expectorating sputum is presented by now. A principle of Cough Assist machine for expectorating sputum provided by Royal Philips is to provide the airway high positive pressure ventilation, which is rapidly converted into negative pressure suction, such that sputum deep down inside lung is discharged by simulating airflow during cough.
However, it is impossible to use the Cough Assist machine as a regular means for expectorating sputum due to requirement to disconnect from the ventilator when using, which is inconvenient and has a high risk for a patient suffering from severe illness. Besides, the Cough Assist machine finishes inhaling by taking time as a control variable of periodic cycle; however, it is safer and more effective to take volume or flow as the control variable of periodic cycle. In addition, it is impossible for the Cough Assist machine to maintain the patient's exhaling to be a positive end expiratory pressure (PEEP), thereby being inappropriate for a patient who needs high PEEP, because such these patients are in danger of alveolar collapse if without the high PEEP. As to another obvious defect, the Cough Assist machine is not provided with an alarming system, which is necessary to a life support system. Finally, as the Cough Assist machine is provided with a pipeline only shared by exhaling and inhaling, it is possible to inhale secretions expectorated previously during next inhaling, which may increase a risk of secondary infection due to the patient himself/herself, shorten a product life of the pipeline, and increase a risk of infecting other patients who use this Cough Assist machine next time.
In addition, WO 2007/054829 A2 provides a mechanical in-exsufflation device, which by ventilator inhales airflow and subsequently exhale airflow in a rapid and short manner for simulating human cough, such that sputum accumulated in airway and trachea branches can be discharged easily. Such a system controls inhaling and exhaling automatically by detecting the airflow, so as to relieve burden of working force. The sputum is expectorated by several times of intermittent negative pressure exhaling, which not only prevents pulmonary alveoli from collapsing caused by over high negative pressure, but also affects a dozen levels of bronchus, thereby discharging sputum deep down inside lung. However, all movements of human cough includes deeply inhaling, closing glottis, lowering the diaphragm muscle, rapidly contracting the expiratory muscle and abdominal muscle, increasing intrapulmonary pressure, opening glottis, contracting the diaphragm muscle, discharging the airflow with high pressure from lung. Such a device for expectorating sputum simulates airflow during cough; whereas cough-related respiratory muscle is not involved in the movements. For a patient ventilated mechanically, long-term anoxia and systemic nutrition deficiency causes reduction and atrophy of diaphragm muscle fibre, such that activity of the diaphragm muscle is decreased. Firstly, merely simulating movements of airflow during cough, such a device does not intervene PEEP process such that it is impossible to guarantee sufficient tidal volume before cough if without special control, thereby being unable to form effective cough. Secondly, the PEEP process by a ventilator causes mechanical lung injury itself, and such a device for expectorating sputum merely simulates movements of airflow during cough without accompanying movements in accordance with muscle, inappropriate control may cause additional mechanical lung injury. Thirdly, it is bad for discharging sputum by adversely influencing intensity of cough and expectoration movement due to merely depending on the airflow with negative pressure without assisting with contraction of abdominal muscle. Finally, the patient assisted with discharging sputum may be disconnected from ventilator earlier than expecting; however, it may adversely influence a patient suffering from atrophy of diaphragm muscle by earlier disconnecting from the ventilator.